WO96/20148 discloses a production process of ethylbenzene from benzene and ethylene using a zeolite such as MCM-22, MCM-49 or MCM-56 as a highly selective catalyst. According to this reference, the alkylation is carried out in a liquid phase and the benzene/ethylenemolar ratio is 5 to 10 in most cases and is 5.5 in the Examples.
U.S. Pat. No. 5,334,795 also discloses a synthetic process of ethylbenzene from benzene and ethylene using MCM-22 zeolite. However, in the Examples of this reference, the benzene/ethylene molar ratio is at least 4.6.
JP-B-06043346 discloses a process comprising bringing an organic aromatic compound into contact with a C2.about.C20 olefin to form alkylation products in a distillation column reactor containing a structure obtained by immobilizing a fixed-bed acidic catalyst in a packing for distillation, separating the formed alkylation products and unreacted organic aromatic compound and olefin, and taking out the alkylation products from the distillation reactor at a place under the fixed bed. This reference, however, does not disclose employment of a .beta. zeolite. Moreover, in the Examples of the reference, the benzene/ethylene molar ratio of the reaction products recovered from the bottom is at least 3.47 and the production rate of ethylbenzene relative to the weight of the catalyst is as very low as 0.12 to 0.3.
U.S. Pat. No. 5,118,896 discloses a process in which continuous liquid-phase alkylation of a liquid aromatic compound selected from benzene, toluene and xylene using an olefin as an alkylating agent is carried out in the fixed bed of a reactive-distillation type reactor by using a catalyst comprising a crystalline aluminosilicatezeolite, silica and alumina and having a pore volume of 0.25 to 0.35 ml/g, a pore radius of more than 450 .ANG. and aparticle diameter of not more than 1/64 inch. This reference describes an example using a reactive distillation method in which the molar ratio of the aromatic compound to ethylene is 2. In this example, the conversion of ethylene is as very low as 55%.
JP-A-04187647 discloses a process in which alkylation and transalkylation are carried out together on a molecular sieve catalyst for aromatic alkylation and transalkylation in a liquid phase, and the benzene/ethylene molar ratio is 4 or less and may be about 2. In this reference, a .beta. zeolite is mentioned as the catalyst for alkylation and the benzene/ethylene molar ratio is 5.2 in the Examples. The reference discloses a method for attaining a benzene/ethylene molar ratio of 2 under idealized reaction conditions. However, in this method, the above molar ratio value is attained by feeding ethylene in five stages.
WO96-04225 discloses fixed-bed liquid-phase alkylation by gas-liquid descent parallel-flow method carried out in a trickle bed region by the use of a .beta. zeolite as a catalyst. However, this process has a high productivity but is disadvantageous in that the catalytic activity is greatly changed in the initial stage of the reaction and is difficult to control.
JP-A-03181424 discloses liquid-phase alkylation and transalkylation which use a .beta. zeolite as a catalyst, and the molar ratio of an aromatic hydrocarbon to an olefin is 4 or more in the Examples of this reference.
Zeolite catalysts are used as catalysts for alkylation of aromatic hydrocarbons and can be advantageously used as non-corrosive catalysts in place of the conventional catalysts for Friedel-Crafts reaction. Therefore, various proposals have been put forward for the zeolite catalysts.
However, in most conventional processes, alkylation is carried out at a relatively high benzene/ethylene molar ratio. Since such a high molar ratio increases the amount of unreacted benzene to be recycled and hence the trouble of benzene recovery, it is very disadvantageous from an industrial viewpoint. The reasons why alkylation is carried out at a high benzene/ethylene molar ratio in the conventional processes in spite of the above fact are the following three reasons.
The first reason is the limitation of catalytic capability, i.e., a low selectivity of nuclear ethylation. For example, Y type zeolites have been most widely used as alkylation catalysts. The Y type zeolites have been known to give a satisfactory activity and a good selectivity of a desired product but have been unusable by any means at a low benzene/ethylene molar ratio because it was found that at such a low benzene/ethylene molar ratio, the production of by-products such as butylbenzene and diphenylethane becomes remarkable with an increase of the conversion of benzene, resulting in a markedly decreased selectivity of nuclear ethylation.
The second reason is that in the reaction carried out at a low benzene/ethylene molar ratio under a relatively low pressure by one-stage feed, ethylene exists as bubbles substantially at the inlet of a catalyst layer, so that the activity of the catalyst is markedly decreased. For example, JP-A-04187647 discloses employment of a Y zeolite as a transalkylation catalyst and describes the fact that when transalkylation of diethylbenzene was carried out using the Y zeolyte catalyst in a trickle bed reactor substantially containing a gas phase, the conversion of diethylbenzene was markedly decreased in several hours to 24 hours.
As a result of investigation by the present inventors, the following was found: when benzene is ethylated by a fixed-bed ascending-flow method using a Y type zeolite, at a low benzene/ethylene molar ratio under a relatively low pressure, the selectivity of nuclear ethylation is very low and the activity is markedly decreased. It was confirmed that also in a batch reaction using a Y type zeolite, the selectivity and the activity are markedly decreased with an increase of the conversion of benzene. It is conjectured that this decrease is due to the porous structure of the Y type zeolite. That is, when there are compared the Y type zeolite and a .beta. zeolite which have the same oxygen-containing 12-membered structure, the Y type zeolite has large cavities called super-cages at the intersections of pores. It can be speculated that the production of binuclear products (e.g. diphenylethane) as by-products becomes easy in the cavities and causes the decrease of the selectivity and the clogging of the pores with high-molecular weight substances (i.e. the decrease of the activity) at the same time.
JP-A-06508817 discloses alkylation using as a catalyst a mordenite type zeolite having a silica/alumina molar ratio of more than 160 and an index of symmetry of at least 1. The alkylation is carried out in a reactor containing substantially no gas. Most preferably, it is carried out in a completely liquid phase. The reference describes the fact that the substantial presence of a gas causes accumulation of an alkylating agent in the gas to polymerize the alkylating agent, so that the decrease of the selectivity and the inactivation of the catalyst are accelerated. That is, the reference describes the impossibility of carrying out the alkylation using the above-mentioned mordenite type zeolite as a catalyst in a gas-liquid mixed phase substantially containing ethylene bubbles.
Also in the case of MCM-22, MCM-49 and MCM-56 zeolite catalysts, the highly selective catalysts disclosed in WO96/20148, etc., the benzene/ethylene molar ratio is at least 4.6 as described above and moreover this value could be attained only by multi-stage feed of ethylene to be completely dissolved. That is, for inhibiting the deterioration of the catalytic activity, the reaction can be carried out only under conditions under which ethylene is completely soluble in benzene.
For using the above-mentioned catalyst, complete dissolution of ethylene in benzene is necessary. For carrying out the reaction at a low benzene/ethylene molar ratio, multi-stage feed of ethylene or a high pressure for increasing the solubility would be necessary. Therefore, a higher pressure and a higher-order multi-stage feed are required for utilizing low-purity crude ethylene as a starting material, so that ethylene purification becomes indispensable. Thus, the above-mentioned catalyst is not industrially usable in practice.
The third reason is a problem of removing the heat of reaction. Since the reaction is an exothermic reaction, heat is markedly generated when the reaction is completed at a low benzene/ethylene molar ratio. The rise of the temperature of a catalyst layer caused by the heat generation lowers the selectivity of alkylation, and if a liquid phase cannot be maintained, a remarkable decrease of the catalytic activity cannot be avoided. The removal of the heat of reaction is an important problem, and in conventional processes, alkylation has been unavoidably carried out in the presence of a large excess of benzene (at a high benzene/ethylene molar ratio) also from the viewpoint of the heat removal.
In such circumstances, there has been proposed a method for solving the problem of removing the heat of reaction, in order to achieve alkylation at a low benzene/ethylene molar ratio.
JP-A-04502451 or JP-A-04187647 has proposed a method for multi-stage feed of ethylene. However, an apparatus for the multi-stage feed is complicated, and even when the multi-stage feed was conducted, alkylation is carried out at a high benzene/ethylene molar ratio of 3 or more in Examples of the reference. For example, JP-A-04502451 describes the following: benzene is fed to the first reaction zone of an alkylation reactor having at least two reaction zones and a fresh olefin is fed to the entrance of each zone to carry out alkylation, whereby the benzene/olefin molar ratio in the whole reactor is reduced while maintaining the benzene/olefin molar ratio in each zone at a sufficiently high value to prevent the temperature rise, and thus a temperature rise to an abnormal temperature is avoided, resulting in an improved selectivity and an extended life of a catalyst. The reference describes the fact that since the reaction is carried out at a low temperature, a zeolite catalyst can be held in a liquid phase, so that a time required for the regeneration of the catalyst to become necessary can be extended.
WO96/20148 discloses a process using MCM-22, MCM-49 or MCM-56 zeolite as a catalyst. According to this process, an olefin is fed in multiple stages and the heat of reaction is removed by providing one or more stages of cooling. This reference describes the fact that practice of the process at a substantially constant temperature increases the purity of the product and the life of the catalyst.
When such a process is employed, the number of stages of ethylene feed and the number of stages of cooling should be increased for attaining a lower benzene/ethylene molar ratio, resulting in a complicated apparatus.
On the other hand, there has also been proposed a process in which the heat of reaction is removed as a latent heat for evaporation by carrying out reactive distillation. For example, in the above reference JP-B-06043346, a Y type zeolite as a catalyst is wrapped in cloth and packed in the reactor. The products are recovered from the bottom of the reactor. Since the Y type zeolite catalyst is markedly deteriorated in activity in the presence of substantially gaseous ethylene, the contact between the catalyst and ethylene bubbles is avoided by the use of the cloth and it can be speculated that ethylene is in a substantially completely dissolved state in the reaction zone. However, when a Y type zeolite is used as a catalyst, it is by no means easy to carry out the reaction at a low benzene/ethylene molar ratio, also from the viewpoint of maintaining the selectivity. In practice, the lowest benzene/ethylene molar ratio of the products obtained from the bottom is at least 3.47 in the Examples of JP-B-06043346. In such a reactive distillation method, since gaseous ethylene forms a continuous phase and moreover there is employed a catalyst packing method which avoids the contact between the catalyst and ethylene bubbles, complete conversion of ethylene is considered impossible as a matter of course.
U.S. Pat. No. 5,118,896 also discloses a reactive distillation method and mentions employment of a .beta. zeolite as a catalyst. However, only ethylation of toluene by the use of a Y type zeolite is described in the Examples of this reference, and in the Examples using reactive distillation, the catalyst is used wrapped in cloth. Therefore, also in these Examples, ethylene bubbles are shut off from a catalyst, so that a uniform liquid phase is maintained in a reaction zone. In this case, the area of gas-liquid interface is inevitably the surface area of the cloth and hence cannot be large. Therefore, a considerable number of stages (a considerable amount of the catalyst) is necessary for completing the conversion of ethylene, so that the productivity relative to the catalyst would be low. In practice, in the Examples of the reference, the conversion of ethylene is only 55% under the following conditions; (toluene+benzene)/ethylene molar ratio: 2, catalyst weight: 272 g, feed rate of toluene: 151 g/Hr, feed rate of benzene: 27 g/Hr. In addition, carrying out the reaction by the reactive distillation method described above is dis-advantageous in that fixation of the catalyst layer in a reactor is difficult, resulting in a complicated apparatus.
The same reference U.S. Pat. No. 5,118,896 describes ethylation of toluene by the use of a Y type zeolite by a fixed-bed ascending-flow gas-liquid mixed-phase method using ethylene diluted with methane as a starting material, as an experiment for evaluating the catalyst. However, in this case, a catalyst layer is maintained at a constant temperature with a divided electric furnace. In such a reaction method, even if a .beta. zeolite is used as a catalyst in place of the Y type zeolite, complete conversion of ethylene cannot be achieved and the catalyst is markedly deteriorated in activity. The reason is as follows: the starting liquid components are evaporated from the inlet of the catalyst layer at a high temperature and the solubility of ethylene at the inlet of the catalyst layer is decreased, so that the reaction cannot be completed; and the higher the temperature, the more remarkable the deterioration in activity of the catalyst in the presence of gaseous ethylene. Examples of the above reference are only intended to evaluate the catalyst and no long-term operation was carried out therein. In these Examples, even short-term operation resulted in a decrease of the catalytic activity, and no complete conversion of ethylene was achieved.
As described above, in the conventional processes, the reaction is carried out in a complete dissolution system in order to prevent substantially the presence of ethylene bubbles at least in a reaction zone, in view of the capability of a catalyst (the selectivity), the decrease of activity of the catalyst, and the removal of the heat of reaction. That is, there has been no choice but to carry out the reaction at a high benzene/ethylene molar ratio.
In such circumstances, when an industrially advantageous low benzene/ethylene molar ratio is desired, the reaction should be carried out under a very high pressure, for example, for attaining the low benzene/ethylene molar ratio in one stage in a complete dissolution system. Therefore, in the conventional processes, there is no choice but to employ a method comprising multi-stage feed of ethylene. Since there is also a problem of removing the generated heat of reaction, a heat exchanger should be provided. If the heat of reaction is not removed, the temperature of a catalyst layer is raised, so that the solubility of ethylene would be further decreased. For carrying out the reaction in a complete dissolution system, starting ethylene is required to have a high purity and hence ethylene purification is absolutely necessary. This is because a higher pressure is required for using diluted ethylene as a starting material.
On the other hand, there have often been proposed so-called reactive-distillation type reactors in which latent heat of evaporation is utilized for removing the heat of reaction. However, also in the actual reaction zone of such a reactor, ethylene should be completely dissolved and avoidance of the contact between a catalyst and ethylene bubbles is important, resulting in a complicated apparatus and a complicated method for packing the catalyst. Moreover, since complete conversion of ethylene is impossible in principle, a finishing reactor is necessary for converting the residual ethylene as much as possible. Such demands on an apparatus pose an important problem in industrial production.